1. Field of the Invention
The subject invention relates to an electrically driven light-emitting-diode (LED) lamp assembly and a method of fabrication.
2. Description of the Prior Art
Various LED thermal management systems have been devised to improve the heat transfer from the LED to external heat dissipaters. Such systems typically include an electrically insulating circuit board having opposed first and second surfaces with electrically conductive circuit traces on the first surface of the circuit board. A plurality of LEDs have a pair of electrical leads in electrical engagement with the traces. A heat dissipater is disposed in parallel relationship to the circuit board. Examples of such systems are disclosed in U.S. Pat. Nos. 5,785,418; 5,857,767 and 6,045,240 to the inventor named herein.
Conventional LEDs exhibit substantial thermal resistance because of poor thermal coupling with the heat dissipater. That is, the LED is unable to efficiently transmit heat through the leads and into the heat dissipater. Some improvement in thermal performance is possible by making the leads of copper rather than steel. However, mounting problems prevent full advantage of the highly conductive leads. Since most the leads of most LEDs are soldered to the traces on the circuit board, the heat from the soldering process can easily damage the LED if the leads are highly thermally conductive. The aforementioned U.S. Pat. No. 5,857,767 addresses this problem by adhesively attaching the leads of the LED to the traces. Conventional LEDs employ two electrical leads which also serve as the heat conduction path to a heat dissipater. The difficulty in fabricating LEDs that cannot be soldered has resulted in the development of a new class of LEDs which separates the thermal path from the electrical path. Such LEDs are characterized by having an integral heat sink separate from the electrical leads.
When operated at the higher currents typical for such advanced LEDs (on the order of 250 mA) the forward voltage drop across the device (typically 2.5 volts) rises significantly above the nominal 1.85 volts of the same device operated at more normal currents of 25 mA per device. This non-linear increase in device dissipation at higher power levels makes efficient heat removal a priority. Heat rejection from the LED is particularly important as the luminous output may decrease approximately 1% per degree C. in temperature rise, and permanent degradation (aging) of LED is an exponential function of operating temperature.
As an example, the highly capable Hewlett Packard/Lumileds high flux emitter is commonly mounted onto a metal core printed circuit board, as noted in Hewlett Packard data sheets for the HPWL-MDXX family of devices. Examination of this product shows adhesive attachment of each LED in the array to a laminate (circuit board) which is then adhesively bounded on to an aluminum heat dissipater or heat sink. However, the inherently poor thermal conductivity of the laminate (circuit board) acts as an impediment to the efficient removal of heat from the LED.
The subject invention provides an electrically driven light-emitting-diode (LED) lamp assembly comprising an electrically insulating circuit board having opposed first and second surfaces with electrically conductive circuit traces on the first surface of the circuit board. A plurality of LEDs are included with each LED having a heat sink and a pair of electrical leads in electrical engagement with the traces. A heat dissipater is disposed in parallel relationship to the circuit board. The assembly is characterized by the circuit board presenting a hole therethrough and around each LED with the heat sink of each LED being disposed in thermal contact with the heat dissipater for conveying heat from the LEDs to the heat dissipater.
The invention also includes a method of fabricating the assembly characterized by disposing each LED in the hole through the circuit board and around each LED with the heat sink of each LED being disposed in thermal contact with the heat dissipater for conveying heat from the LEDs to the heat dissipater.
Accordingly, the subject invention provides an unique combination wherein the integral heat sink of an LED has direct thermal conductivity with the heat dissipater, either by direct contact or through a thermal coupling.